Certain techniques have been developed to extract hydrocarbons that involve drilling a wellbore in close proximity to one or more existing wells. Examples of directed drilling near an existing well include well intersection for blowout control, multiple wells drilled from an offshore platform, and closely spaced wells for geothermal energy recovery. Another such technique is steam-assisted gravity drainage (SAGD) that uses a pair of vertically-spaced, horizontal wells constructed along a substantially parallel path, often less than ten meters apart. Careful control of the spacing contributes to the effectiveness of the SAGD technique.
Several methods have been developed for use in SAGD applications to utilize electrical currents along a target well (existing cased-hole well) in order to achieve range determination. Such ranging service provides distance and direction between the target well and a drilling well (ranging well). The goal is to range a wellbore of the drilling well to an optimized position relative to the target well. For example, gradient sensors implemented in the wellbore of the drilling well have been proposed to provide distance and direction of the target well without knowing amplitudes of electrical current signals in the target well.